High-stength bolting of steel structures have surpassed the use of rivots because of distinct economic advantages. Though the purchase price of high-strength bolt is nearly three times that of a rivot, the overall cost of the bolted construction is cheaper than the rivoted construction because of the reduced labor and equipment costs and the reduced number of bolts required to resist the same loads. Other advantages include the use of lesser skilled labor and the ability to erect rapidly structures in the field.
The installation of the high-strength bolts consists of tightening the bolt to a minimum tension. The reason that minimum tension is desired is to prevent the occurrence of bolt deflection when actual loads are applied. Bolt deflection will cause eccentric load to bear which subjects the bolt to possible shearing and bending moments for which it may not be designed.
Presently, the methods most commonly applied to obtain minimum tensile stength include: (1) applying a determined torque to each size bolt and having the wrench stall automatically (or "breaking the bolt") at a predetermined torque, (2) by back-turning the bolt a determined amount from a snug-tight position known as "turn of the nut" method, and by (3) tightening a washer having bumps and then flattening them by rotating a nut or bolt unto the washer, thus, indicating the tension applied.
None of the above stated methods is satisfactory in obtaining desired tensile strength. There serveral reasons why these methods are unsatisfactory. First, visual inspection is required at the time of installation to check that the carefully calibrated tools required are used correctly. Second, workers are oftentimes known to flatten the bumps on tension washers with hammers prior to installation to make the bolt easier to tighten thereby defeating the tension washer's ultimate purpose. A third reason is that rusty twist off bolts may break off before they reach the right tension. Fourth, none of these methods indicates if the bolt has been loosened due to temperature variation or vibration.
Furthermore, these methods of installation do not indicate overtightening. The concerns of overtightening the bolts to the extent that they reach a plastic range are grave. Some argue that overtightening is not a problem because if a bolt is overtightened, it would break and be replaced. However, replacement is very costly with respect to anchor bolts that would have the broken piece embedded in the connecting element.